Provide a default logging implementation

[asgrim]

#117 changes requirement to require a \Psr\Log\LoggerInterface explicitly into the agent, but the suggestion for a default is NullLogger.

We could provide a very basic logger implementation that writes to file for those who are not familiar with PSR-3 and suggest that as a default.

[asgrim]

Thoughts welcome on this...

[asgrim]

@cschneid said:

we do exactly that in ruby, although ruby provides 99% of the impl
python we rely on the native logger
ruby agent by default looks for a APP_ROOT/log/scout_apm.log file to write to
a log in /tmp with the app name would work